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Incontinentia pigmenti (IP) is an X-linked-dominant Mendelian disorder caused by mutation in the IKBKG/NEMO gene, encoding for NEMO/IKKgamma, a regulatory protein of nuclear factor kappaB (NF-kB) signaling. In more than 80% of cases, IP is due to recurrent or nonrecurrent deletions causing loss-of-function (LoF) of NEMO/IKKgamma. We review how the local architecture of the IKBKG/NEMO locus with segmental duplication and a high frequency of repetitive elements favor de novo aberrant recombination through different mechanisms producing genomic microdeletion. We report here a new microindel (c.436_471delinsT, p.Val146X) arising through a DNA-replication-repair fork-stalling-and-template-switching and microhomology-mediated-end-joining mechanism in a sporadic IP case. The LoF mutations of IKBKG/NEMO leading to IP include small insertions/deletions (indel) causing frameshift and premature stop codons, which account for 10% of cases. We here present 21 point mutations previously unreported, which further extend the spectrum of pathologic variants: 14/21 predict LoF because of premature stop codon (6/14) or frameshift (8/14), whereas 7/21 predict a partial loss of NEMO/IKKgamma activity (two splicing and five missense). We review how the analysis of IP-associated IKBKG/NEMO hypomorphic mutants has contributed to the understanding of the pathophysiological mechanism of IP disease and has provided important information on affected NF-kB signaling. We built a locus-specific database listing all IKBKG/NEMO variants, accessible at http://IKBKG.lovd.nl., (© 2013 WILEY PERIODICALS, INC.)

The Incontinentia Pigmenti (IP) locus contains the IKBKG/NEMO/IKKgamma gene and its truncated pseudogene copy, IKBKGP/deltaNEMO. The major genetic defect in IP is a heterozygous exon4_10 IKBKG deletion (IKBKGdel) caused by a recombination between two consecutive MER67B repeats. We analyzed 91 IP females carrying the IKBKGdel, 59 of whom carrying de novo mutations (65%). In eight parents, we found two recurrent nonpathological variants of IP locus, which were also present as rare polymorphism in control population: the IKBKGPdel, corresponding to the exon4_10 deletion in the pseudogene, and the MER67Bdup, that replicates the exon4_10 region downstream of the normal IKBKG gene. Using quantitative DNA analysis and microsatellite mapping, we established that both variants might promote the generation of the pathological IKBKGdel. Indeed, in family IP-516, the exon4_10 deletion was repositioned in the same allele from the pseudogene to the gene, whereas in family IP-688, the MER67Bdup generated the pathological IKBKGdel by recombination between two direct nonadjacent MER67Bs. Moreover, we found an instance of somatic recombination in a MER67Bdup variant, creating the IKBKGdel in an IP male. Our data suggest that the IP locus undergoes recombination producing recurrent variants that might be "at risk" of generating de novo IKBKGdel by NAHR during either meiotic or mitotic division.

Mutations in the inhibitor of kappa light polypeptide gene enhancer in B-cells, kinase gamma (IKBKG), also called nuclear factor-kappaB (NF-kB) essential modulator (NEMO), gene are the most common single cause of incontinentia pigmenti (IP) in females and anhydrotic ectodermal dysplasia with immunodeficiency (EDA-ID) in males. The IKBKG gene, located in the Xq28 chromosomal region, encodes for the regulatory subunit of the inhibitor of kappaB (IkB) kinase (IKK) complex required for the activation of the NF-kB pathway. Therefore, the remarkably heterogeneous and often severe clinical presentation reported in IP is due to the pleiotropic role of this signaling transcription pathway. A recurrent exon 4_10 genomic rearrangement in the IKBKG gene accounts for 60 to 80% of IP-causing mutations. Besides the IKBKG rearrangement found in IP females (which is lethal in males), a total of 69 different small mutations (missense, frameshift, nonsense, and splice-site mutations) have been reported, including 13 novel ones in this work. The updated distribution of all the IP- and EDA-ID-causing mutations along the IKBKG gene highlights a secondary hotspot mutation in exon 10, which contains only 11% of the protein. Furthermore, familial inheritance analysis revealed an unexpectedly high incidence of sporadic cases (>65%). The sum of the observations can aid both in determining the molecular basis of IP and EDA-ID allelic diseases, and in genetic counseling in affected families.